1. Field of the Invention
The present invention relates to a resin-filled ferrite carrier core material and a ferrite carrier for an electrophotographic developer, being used in apparatuses such as copiers and printers, being excellent in durability because of having a light true density and a high carrier strength, and causing no charge variation at the time of endurance printing, and an electrophotographic developer using the ferrite carrier.
2. Description of the Related Art
An electrophotographic development method is a method in which development is performed by adhering the toner particles in a developer to the electrostatic latent image formed on a photoreceptor, and the developer used in such a method is classified into a two-component developer composed of toner particles and carrier particles and a one-component developer using only toner particles.
As a development method using, among such developers, a two-component developer composed of toner particles and carrier particles, previously a method such as a cascade method has been adopted, but currently a magnetic brush method using a magnet roll predominates.
In a two-component developer, the carrier particles serve as a carrying substance to form a toner image on the photoreceptor in such a way that the carrier particles are stirred together with the toner particles in a developer box filled with the developer to impart an intended charge to the toner particles, and further, convey the thus charged toner particles to the surface of the photoreceptor to form the toner image on the photoreceptor. The carrier particles remaining on a development roll which holds a magnet again return from the development roll into the developer box to be mixed and stirred with the fresh toner particles and to be repeatedly used for a predetermined period of time.
In contrast to a one-component developer, a two-component developer is such that the carrier particles are mixed and stirred with the toner particles, thus charge the toner particles, and further have a function to convey the toner particles, and a two-component developer is excellent in the controllability in designing developers. Accordingly, two-component developers are suitable for full-color development apparatuses required to offer high image quality and for high speed printing apparatuses required to be satisfactory in the reliability and durability in image maintenance.
In two-component developers used in the above-described manner, the image properties such as the image density, fogging, white spots, gradation and resolution are each required to exhibit a predetermined value from the initial stage, and further these properties are required to be invariant and to be stably maintained during the endurance printing. For the purpose of stably maintaining these properties, the properties of the carrier particles contained in the two-component developers are required to be stable.
As the carrier particles forming two-component developers, there have hitherto been used various carriers such as iron powder carriers, ferrite carriers, resin coated ferrite carriers and magnetic powder-dispersed resin carriers.
Recently office networking has been promoted, and the age of monofunctional copiers develops into the age of multifunctional copiers; the service system has also shifted from the age of the system such that a contracted service man conducts periodic maintenance inclusive of the replacement of the developer to the age of the maintenance-free system; thus, the market has further enhanced demand for further longer operating life of the developer.
Under such circumstances, for the purpose of reducing the carrier particle weight and extending the developer operating life, there have been also proposed a variety of magnetic powder-dispersed carriers in each of which magnetic fine particles are dispersed in a resin in Japanese Patent Laid-Open No. 5-40367 etc.
Such magnetic powder-dispersed carriers can be reduced in true density by decreasing the amounts of the magnetic fine particles and can be reduced in stress caused by stirring, and hence can be prevented from the abrasion and exfoliation of the coating film and accordingly can offer stable image properties over a long period of time.
However, the magnetic powder-dispersed carrier is prepared by agglomerating magnetic fine particles with a binder resin, and hence offers, as the case may be, a problem that the magnetic fine particles are detached due to the stirring stress or the impact in the developing device or a problem that the carrier particles themselves are cracked probably because the magnetic powder-dispersed carriers are inferior in mechanical strength to the iron powder carriers and ferrite carriers having hitherto been used. The detached magnetic fine particles and the cracked carrier particles adhere to the photoreceptor to cause image defects as the case may be.
Additionally, the magnetic powder-dispersed carrier uses magnetic fine particles, and accordingly has a drawback that the residual magnetization and the coercive force are high and the fluidity of the developer is degraded. In particular, when a magnetic brush is formed on a magnet roll, the presence of the high residual magnetization and the high coercive force hardens the ears of the magnetic brush and hence high image quality is hardly obtained. Also, even when the magnetic powder-dispersed carrier is separated away from the magnet roll, the magnetic coagulation of the carrier is not unstiffened and the mixing of the carrier with the supplied toner is not rapidly conducted, and hence there occurs a problem that the charge amount rise is aggravated, and image defects such as toner scattering and fogging are caused.
In addition to such magnetic powder-dispersed carriers, for the reduction of the weight of the carrier particle, there have been proposed hollow carriers in which a vacancy is formed in the interior of the carrier core material particle. For example, Japanese Patent Laid-Open No. 2008-310104 states that a core particle has at least a vacancy of 20% or more and 65% or less in terms of the cross sectional area, and the overall vacancy proportion in terms of the cross sectional area is 20% or more and 70% or less. Japanese Patent Laid-Open No. 2009-244572 states that when the outer diameter of the carrier core material is represented by d1 and the outer diameter of the vacancy present in the interior of the core material is represented by d2, the relation 0.1<d2/d1<0.9 is preferably satisfied.
In the carriers described in these patent documents, the weight reduction is certainly attained; however, in any of these carriers, the size of one vacancy is extremely large, and hence, as compared to conventional ferrite carriers having no hollow portion, these carriers are still weak in mechanical strength, thus the fracture of the carrier particles occurs due to the stirring stress or the impact in the developing device at the time of endurance printing, and the fractured particles adhere to the photoreceptor to offer a cause for the occurrence of image defects. Accordingly, for the extension of the operating life having been recently, particularly demanded, these carriers are absolutely unsatisfactory.
Further, as the substitutes for such magnetic powder-dispersed carriers and hollow carriers, resin-filled ferrite carriers obtained by filling a resin in the voids of ferrite carrier core materials using porous ferrite particles have been proposed.
Japanese Patent Laid-Open No. 2006-337579 proposes a resin-filled ferrite carrier prepared by filling a resin in a ferrite carrier core material having a porosity of 10 to 600, and Japanese Patent Laid-Open No. 2007-57943 proposes a resin-filled ferrite carrier having a three-dimensional laminated structure. Further, Japanese Patent Laid-Open Nos. 2009-175666 and 2009-244837 each specify the pore volume, the pore size and the pore size distribution property of the ferrite carrier core material including porous ferrite particles to be filled with a resin, and propose a resin-filled ferrite carrier which has a high insulation breakdown voltage and is improved in the fracture strength of the carrier particle, and a resin-filled ferrite carrier which is fast in the charge rise property and causes no charge variation, respectively.
In each of the resin-filled ferrite carriers described in these patent documents, a resin is filled even in the interior of the porous ferrite particles to form a three-dimensional laminated structure. In particular, in Japanese Patent Laid-Open Nos. 2009-175666 and 2009-244837, the pore size distribution property is controlled more accurately, and hence the variation of the resin filling degree is reduced, and further, it is stated that the surface of the filling resin is preferably coated with a resin. Consequently, indeed the weight reduction of the carrier particle is attained, and the carrier strength is improved to a certain degree; however, these carriers are far from having a sufficient carrier strength. Accordingly, in particular, for the high durability having been recently demanded, these carriers are far from being satisfactory.
On the other hand, Japanese Patent Laid-Open No. 2007-271663 describes a ferrite carrier for an electrophotographic developer having a compression fracture strength of 150 MPa or more and a compression variation rate of 15.0% or more, and states that the carrier is excellent in the strength against the fracture due to stress when used as a developer.
However, the ferrite carrier (ferrite particles) used in Japanese Patent Laid-Open No. 2007-271663 is not porous, and is not a resin-filled ferrite carrier using porous ferrite particles, and hence it is impossible to obtain an advantage of a resin-filled ferrite carrier, such as the weight reduction.
Therefore, with respect to the demand for high durability, there has been demanded a resin-filled ferrite carrier for an electrophotographic developer in which, while the weight reduction is being achieved, the improvement of the carrier strength is achieved, and the charging property is stabilized at the time of endurance printing.